The present invention relates to the field of image capture devices, in particular images containing coded optical information. More in particular, the invention relates to an image capture module for such a device, comprising a liquid lens. The invention also relates to an electro-optical element for such an image capture module, and an image acquisition device which comprises such a module.
In the present description and attached claims, under “coded optical information” or in brief “optical code” it is intended to mean any graphical representation having the function of storing coded information by means of a suitable combination of elements of pre-established shape, for examples squares, rectangles or hexagons, of dark colour (normally black) separated by light colours (spaces, normally white), such as bar codes, stacked codes, i.e. with several superimposed bar sequences, and two-dimensional codes in general, colour codes, etc., as well as alphanumeric characters and particular shapes or patterns such as stamps, logos, signatures etc.
The expression “image” and in particular “optical code” comprises graphical representations detectable not only in the visible light range, rather also in the wavelength range comprised between infrared and ultraviolet. In the present description and attached claims, under “light” it is intended to generally mean any radiation suitable for capturing an image, and in particular for detecting an optical code.
The capture devices of images containing coded information are commonly known as optical code readers of the imager type, in particular bar code readers of the imager type.
Falling within the scope of the invention are imager readers capable of capturing pictures or films and/or capable of capturing images of documents for automatic character recognition or for “document handling” applications.
Such imager devices comprise an image capture module generally comprising a linear or matrix photo-sensitive device or pixel array sensor, and optics for focusing the light onto the pixel array sensor.
Besides a resolution limit in terms of pixels, dictated by the pixel array sensor, any imager device has as a further limit its own depth of field or DoF, which is the distance range at which the device is able to capture the image with suitable focusing, depending on the optics. It is manifest that in the case of imager devices of both the manual and the fixed type, such as for example bar code readers on conveyors used for example in applications at airports for sorting luggage, there is often the need to increase the depth of field.
A first expedient for increasing the depth of field consists of providing several readers or image capture modules having respective optics differently focused. In U.S. Pat. No. 7,195,164, one of the image capture modules mounted inside a reader can in turn have variable focus. Providing for more than one image capture module is nevertheless costly and creates a complex architecture that is hard to manage.
Several variable, in particular automatic, focusing systems are well known. The most common and simple, used for example in cameras and video cameras, is based on an electric motor that, by physically moving the optics or a part thereof, changes the focal length of the system. Typically, an objective is provided comprising a first fixed group of lenses (fixed or primary optics) and a second group of lenses, moved by the motor. The second group of lenses is called the afocal part of the objective, since it is not capable on its own to form an image onto a plane, and must always be coupled with primary optics.
One such focusing system is described for example in U.S. Pat. No. 7,303,131, U.S. Pat. No. 6,431,452 and U.S. Pat. No. 5,378,883. The suitable focal length can be established through a measurement of the distance which separates the reader from the support or target on which there is the bar code to be read. The distance measurement method proposed in such documents is based on the use of a laser pointer, with which the reader is provided. If the distance measurement fails, the system identifies the desired focus by comparing different sample images taken with different focusing conditions.
In U.S. Pat. No. 7,222,793, an electric motor does not move a lens, rather a mirror, so as to direct a light beam along different optical paths, characterised by different focal lengths. The focusing system therefore allows preset and selectable focal lengths to be obtained.
Recently, technology started to offer actuators different from the traditional electric motors, which can be used for mechanically moving parts of the optics of an imager device. “Voice coils”, i.e. motors which exploit sound wave propagation for mechanically moving axes, as well as piezoelectric actuators, are part of this class. For example, in U.S. Pat. No. 7,083,096 a piezoelectric actuator is used for moving a lens group and obtaining the best focus position; the method used for deciding the positioning is based on distance measurement. In U.S. Pat. No. 6,634,554, a focusing system is described in wherein a piezoelectric actuator modifies the deflection angle of a mirror, thus changing the optical path and thus the focusing length of the system.
However, voice coils are bulky devices. Piezoelectric actuators are costly, rather noisy, are not very reliable, and have an insufficiently long operating lifetime. A piezoelectric motor rarely reaches a million cycles, while a bar code reader is typically used a few thousands times a day, and thus reaches a million cycles already during its first operating year.
Moreover, the focus variation, in particular autofocus, systems based on the movement of parts typically have relatively long response times. Providing only two selectable focusing positions, as described for example in U.S. Pat. No. 7,073,715, allows obtaining a faster response of the system; however the response is still poorly suited, in particular for coded information readers.
Indeed, an important factor to be taken into account in the field of optical code readers is the decoding time, i.e. the time that elapses between the activation of the reader, whether caused by a human operator pressing the trigger or by an automatic system, and the decoding. During such decoding time, the correct focusing of the system must occur, among other things. In the case of manual readers, this time should not exceed six hundred milliseconds, since beyond such time the operator perceives the reader as being very slow. In the field of automatic optical code readers, the decoding time should be even shorter, on the order of a few millisecond units.
In order to obtain a quick response of the focusing system, in particular in the field of optical code readers, liquid lens capture modules have recently been developed.
In brief, as shown in FIG. 1, a liquid lens 400 comprises two immiscible fluids 401, 402 in contact with each other, one being a conductor 401 and one being an insulator 402, having different refractive indices. The shape of the interface 403 (meniscus) between the two fluids, and consequently the optical path through the liquid lens 400, can be changed by means of some variables, such as for example the quantity of the two fluids 401, 402, the pressure applied thereto, and, of particular interest with regard to the invention, the voltage applied to its electrodes 404, 408, one of which being in contact with the conductor fluid. The shape variation of the interface 403 and consequently of the optical path can be controlled to change in particular the dioptric power, namely the focal length, of the lens 400.
Further details on liquid lenses can be found in U.S. Pat. No. 6,369,954 B1, which describes their architecture, and in US 2008/0204891 A1, related to driving methods aimed at obtaining a quick response of the liquid lens to electrical bias, incorporated herein by reference.
In US 2007/0131770 A1, a digital image capture device is disclosed, with two focus positions obtained by means of a liquid lens. The selection between the two focus positions is operated, for example, based on a distance measure. The control of the two focus positions is of so-called open loop type, i.e. no measurement is carried out on the effective attainment of the desired focus condition, moving instead from the assumption that the system parameters are sufficiently repeatable and reliable.
US 2007/0063048 A1 teaches to carry out a calibration of the drive signal of a liquid lens reader in order to compensate for the effects on the behaviour of the specific liquid lens of various factors such as temperature, ambient pressure, ageing of the fluids, vibrations and accelerations, etc.
However, the latest generation liquid lenses are very stable and hence the temperature compensation can be omitted, in particular when the drive signal is feedback controlled by focus condition achievement indexes.
Such document moreover discloses a drive circuit for the liquid lens, comprising a direct voltage generator with adjustable output and an H-bridge circuit having four transistor switches, of the FET type. A switch circuit controls the state of the switches so as to alternately close the pairs of switches arranged in opposite branches of the bridge, thus driving the liquid lens by means of a square wave voltage. Alternatively, the document teaches driving by means of square waves generated by an integrated drive circuit such as those used for electroluminescent lamps in mobile phones.
Similarly, liquid lens manufacturers propose drive schemes based on integrated circuits designed for generating, by means of an H-bridge, the high voltage necessary for driving a power load such as an electric motor or an electroluminescent lamp for back-lighting displays. The four switches of the H-bridge must therefore be capable of conducting a current of relatively high value, necessary for driving such power load. When implemented with solid-state switches, two N-mos transistors and two P-mos power transistors may be used. The latter must however be driven by a gate voltage on the order of magnitude of the voltage applied to the source, i.e. tens of Volts, which leads to a circuitry complication which has as a consequence an increase of costs and circuit's size.
Moreover, the communication between such general purpose integrated circuits and the microprocessor which controls the focusing of the liquid lens typically occurs through serial interfaces of SPI or I2C type, which add costs, size and complexity to the image capture module.
From what stated above, it appears that the liquid lens technology is sufficiently developed for their successful use as optical element with variable focal length in image capture devices, in particular in coded information readers, and more in particular in imager readers of 1D or 2D bar codes.
For effectively assembling a liquid lens in an image capture device, it is necessary to make a physical connection between the electrodes of the liquid lens itself and the drive circuit.
As shown in FIG. 1, a liquid lens 400 comprises a generally cylindrical hermetically sealed casing, containing the aforementioned two fluids 401, 402. A first electrode 404 typically consists of the lateral wall 405 of the casing and of a peripheral portion 406 of a first base face 407 of the casing, while a second electrode 408 is typically annular, leading to a second base face 409 of the casing opposite the first base face 407. Both electrodes 404, 408 leave a central region 410, 411 of the base faces 407, 409 of the liquid lens 400 free, which acts as an aperture for the passage of the light. The hermetical sealing of the casing of the liquid lens 400 at the apertures 410, 411 is ensured by plates 412, 413 made of a material transparent to the used light wavelength. An insulator 414 separates the two electrodes 404, 408.
In the aforementioned document US 2007/0063048 A1, the liquid lens is supported in a barrel by means of a retention ring screwed in the barrel, and contacting the lens is made by means of lead wires, and possibly by means of a conductive elastomeric O-ring.
US 2008/0037973 A1, which aims at supplying a compact and economical assembly, in order to avoid the direct connection of lead wires to the electrodes of the liquid lens, which would require their insulation, provides for using a barrel supporting the lens, and a housing coupled to the barrel and bearing a sensor. First and second elastic electrodes are provided on the barrel for contact with the electrodes of the liquid lens, and third and fourth electrodes are provided on the housing for contact with the first and second electrodes and with power supply terminals provided on the sensor.
Also US 2008/0239509 A1 aims at a very compact assembly of an image capture module, which provides for a liquid lens. The module comprises a barrel which supports a plurality of fixed focal length lenses and a diaphragm positioned in front of the lenses. Support arms for a liquid lens extend from the diaphragm. The barrel is fixed in a housing which supports a sensor. The contact with a voltage generator for driving the liquid lens is schematically shown as achieved by means of leads outside the housing.
Providing for a diaphragm in an image capture module contributes to determining its F-number f/#=EFL/EPD, wherein EFL represents the focal length of the lens or the lens system, and EPD the diameter or more generally the minimum size of the clear aperture.
A module with a small f/# captures bright images, but is characterised by a limited depth of field. Hence, in the field of autofocus systems, it involves the focal length being changed quite often, even for minimal changes of the distance of the module from the target. On the other hand, a large f/# provides a lower brightness of the images that can be captured, but a greater depth of field, thus reducing the operation of the autofocus system, and therefore increasing the response speed of the image capture module, and in particular the decoding speed in the case of optical code readers.
Therefore, the clear aperture must have an optimal diameter for the intended application, for the typical brightness level of the operating conditions, and for the desired depth of field.
Although liquid lenses per se have a relatively small aperture, which could be considered sufficiently small so as to represent the clear aperture of the image capture module, in most image capture module applications it is proper to provide for a diaphragm.
Although diaphragms with variable aperture are known, the diaphragms used in the image capture modules for optical code readers typically have a fixed aperture, so as to limit costs and complexity.
Starting from the above-mentioned state of the art, and in particular from document US 2008/0239509 A1, the technical problem at the basis of the present invention is to improve the integration of a liquid lens in an image capture module, in particular for an image capture device and even more in particular for an optical code reader of the imager type.
In a first aspect thereof, the invention relates to an image capture module, comprising a liquid lens having a first and a second electrode, a first and a second conductor element in electrical contact with said first and second electrode, respectively, said first and second conductor being each intended for connection with a voltage generator for driving the liquid lens, characterised in that the first conductor element comprises an electrically conductive body having a peripheral region for contact with the first electrode of the liquid lens, and having a light diaphragm aperture in a central region thereof.
Such diaphragm aperture corresponds to the clear aperture (or minimum aperture) of the image capture module.
In the present description and attached claims, under the expression “in contact with” or “for contact” it is intended to mean an electrical contact relationship, but not necessarily with direct physical contact.
By combining the optical function of diaphragm element and the electrical function of contacting the liquid lens in a same electro-optical component, the image capture module according to the invention results extremely compact and economical.
In addition, providing such a single component increases the reliability and repeatability of the assembly, since the electro-optical element, due to its greater size, is more easy to handle than a lead wire. Welding of a lead wire directly on the casing of the liquid lens at a first electrode thereof, that could potentially damage it, is also avoided.
The diaphragm aperture is typically rectangular or circular, but can have more complex shapes.
Preferably, the image capture module further comprises a barrel adapted to support the liquid lens and at least said first conductor element in accordance with a pre-established geometric relationship.
Providing such a barrel allows further increasing the reliability and repeatability of the assembly.
The barrel can act as the second conductor element, or it can support a separate component which acts as said second conductor element.
The pre-established geometric relationship is typically an axial alignment relationship.
Preferably, the second conductor element comprises an electrically conductive body having a peripheral region for contact with the second electrode of the liquid lens, and having a central aperture of size not less than that of said diaphragm aperture.
The second conductor element therefore only has electrical contact function, and its geometry is such that it does not hinder the passage of the light through the image capture module. As for the rest, such a second conductor element offers the same advantages in terms of assembly reliability and repeatability of the electro-optical diaphragm and contact element.
Even more preferably, the barrel comprises a notch, and at least the first of said conductor elements comprises a respective protrusion extended into said notch.
By means of such a provision, the function of leading the electrical contacts outside the barrel, and thus outside the optical part of the module, is combined with that of presetting the orientation in the plane transversal to the optical axis of the, and in particular of the first, conductor elements, further facilitating the assembly reliability, repeatability and simplicity.
Preferably, at least the first of said conductor elements is rigid, so as to prevent misalignments of the liquid lens.
Typically, at least the first of said conductor elements is made of beryllium copper, which has good electrical conductivity and high machinability precision characteristics, such as to allow the creation of a highly defined diaphragm aperture. The conductor elements can be made of other materials having such good electrical conductivity and, as far as the first conductor element is concerned, machinability high precision characteristics.
Preferably, at least one of said conductor elements has a size in the direction of an optical axis of the liquid lens greater than 0.1 mm, in order to further act as a spacer between the liquid lens and the adjacent component of the image capture module.
Typically, the image capture module further comprises fixed optics, comprising one or more lenses, preferably housed in said barrel.
Preferably, the image capture module further comprises a ring for locking the liquid lens, the conductor element(s), and possibly the fixed optics within the barrel.
The contact between the first conductor element and/or the second conductor element, and the electrodes of the liquid lens can occur only by adjoining and compressing by means of the locking ring, but preferably at least the first of said conductor elements is fixed to the respective electrode of the liquid lens by means of conductive glue, conductive springs or conductive spacers.
Typically, the image capture module further comprises a linear or two-dimensional pixel array sensor.
Typically, the pixel array sensor is selected from the group consisting of a CCD sensor, and a C-MOS sensor.
The pixel array sensor is preferably fixed to the barrel in a predetermined geometric relationship with respect to the liquid lens, typically in an axial alignment relationship.
The first conductor element can be arranged upstream or downstream of the liquid lens with respect to the pixel array sensor.
Preferably, at least the first conductor element of the image capture module is connected to the drive voltage generator circuit by means of sliding contact or direct welding of a protrusion thereof, wiring with a welded wire, at least one conductive material spring, conductive glue or conductive paint.
Preferably, the image capture module further comprises a drive circuit of said liquid lens, comprising an H-bridge circuit configured for supplying the liquid lens with a cyclic wave voltage signal, wherein the H-bridge circuit comprises two resistors and two switches, alternately driven one closed and the other one open.
By replacing two of the four switches typically provided in a drive H-bridge of a liquid lens with resistors, considerable advantages in terms of costs and simplicity are obtained.
The Applicant has in fact recognised that a liquid lens, which is substantially a capacitive load, has limited capacitance at the terminals, on the order of a few hundred pF, and absorbs low current, on the order of a few hundred μA.
The Applicant has then perceived that the two power switches of the known H-bridges for driving a liquid lens can be replaced by simple and economical resistors. Indeed, each resistor, to ensure the flow of the necessary current through the liquid lens, can still be sized sufficiently large so as not to short-circuit the H-bridge circuit when the switch in the adjacent branch is closed. The Applicant also has perceived that, due to the low current, the power loss in each resistor when the switch in the adjacent branch of the H-bridge circuit is closed is in any case limited.
More in particular, the H-bridge preferably comprises:                two output terminals, the liquid lens being connected between the two output terminals,        two input terminals, kept at a direct voltage difference of a value comprised within a predetermined range,        a first resistor connected between the first input terminal and the first output terminal,        a second resistor connected between the first input terminal and the second output terminal,        a first switch connected between the first output terminal and the second input terminal, driven closed and open by a first cyclic wave signal,        a second switch connected between the second output terminal and the second input terminal, driven closed and open by a second cyclic wave signal, equal to and in counter-phase with respect to the first cyclic wave signal.        
In the present description and attached claims, under “direct voltage difference” or “direct voltage”, it is intended to mean a substantially direct voltage signal, which can however have comparatively small oscillations (ripples).
Preferably, the two switches are low power, solid state switches.
In the present description and in the attached claims, under “low power” it is intended to mean a power on the order of 100 mW.
Thanks to the provision of high-resistance resistors, the two remaining switches are flown by low currents, and thus can be made by low power solid state switches advantageously drivable by a sufficiently low level signal, as can be directly supplied by a microprocessor.
In an embodiment, the two switches are comprised of N-mos transistors.
In an embodiment, the two switches are comprised of P-mos transistors.
In an embodiment, a control input of the first switch and a control input of the second switch are connected through an inverter.
Preferably, the control input of the first switch is arranged for connection to a terminal of a microprocessor, and said inverter is a digital inverter.
In an embodiment, a control input of the first switch and a control input of the second switch are arranged for connection to respective terminals of a microprocessor.
Preferably, the H-bridge circuit has discrete components.
Preferably, the drive circuit comprises a direct voltage generator.
Preferably, the direct voltage generator has discrete components.
The direct voltage generator is preferably controllable by a level control block, so as to provide a direct voltage difference of an adjustable value.
Preferably, the level control block is arranged for generating a pulse width modulated signal having an adjustable duty cycle.
The drive circuit preferably comprises a level adapter (voltage lowerer) which converts the direct voltage difference to a lower value direct voltage difference, compatible with digital signals.
The low level direct voltage difference can be used for a feedback control of the level of the voltage generator, and optionally of the control signals of the H-bridge switches, so to compensate for voltage generator drifts.
Preferably, the level adapter comprises a resistive voltage divider.
Preferably, the level adapter has discrete components.
Thanks also to the preferred implementation of the drive circuit by means of discrete components and functional blocks implemented by the microprocessor, communication interfaces are not necessary.
The drive circuit of the liquid lens, and in particular its H-bridge circuit, represent per se inventive aspects, which have general application in driving liquid lenses of image capture modules, independently of the provision of the electro-optical contact and diaphragm element described above.
More in general, the drive circuit of the liquid lens according to the invention, and in particular its H-bridge circuit, are advantageous also in the case of a liquid lens image capture module without autofocus or variable focusing, i.e. wherein the liquid lens is used with a drive voltage of constant root mean square value. Practical applications comprise, for example, the provision of a single component, settable in the factory as a short-distance reading module or as a long-distance reading module, as well as image capture modules wherein an initial calibration in the factory allows for variations between one liquid lens and another to be taken into account.
Even more generally, the drive circuit according to the invention, and in particular its H-bridge circuit, are advantageous in all applications where it is necessary to supply a cyclic current to a load which has limited capacitance at its terminals, on the order to some hundred pF, and low currents, on the order of a few hundred μA.
In a second aspect thereof, the invention relates to an electro-optical element for a liquid lens image capture module, comprising an electrically conductive body, having a peripheral region for contact with an electrode of a liquid lens, and having a light diaphragm aperture in a central region thereof.
In another aspect thereof, the invention relates to an image acquisition device comprising a liquid lens image capture module as described above.
Preferably, the image acquisition device is an optical code reader.
Preferably, moreover, the optical code reader comprises a microprocessor arranged for carrying out optical code decoding functions, the microprocessor directly controlling the drive circuit of the liquid lens.